وَاسْتَغْفِرْ لِذَنْبِكَ وَلِلْمُؤْمِنِينَ وَالْمُؤْمِنَاتِ
Seek forgiveness for yourself and for believing men and women. (Muhammad 47:19)
Asking Allah to forgive another person is a basic form of intercessory request. The expression believing men and women is general; restricting it to those who have never committed major sins requires a further argument.
This establishes the legitimacy and general relevance of such prayer, not a guarantee that every individual request will be accepted. Affirmation of intercession and unconditional acceptance for each person are distinct.